Bookmark and Share

Electron crystallography - theory and techniques

[Cowley] J. Cowley

The Summer School on Electron Crystallography organized by the Commission on Electron Diffraction (CED) was held on the campus of Tsinghua U., Beijing, from Aug. 16 to 20. The School covered theory and applications of high-energy electron diffraction. The first two days were devoted to applications where the simple kinematical theory of diffraction could be used. D. Rankin gave an account of gas electron diffraction and its use in determining the structures of gas molecules. This was followed by D. Cockayne's account of the study of amorphous solids, with particular application to Buckyballs. The evening problem-solving session on gas diffraction was very popular. Applications of kinematical diffraction to crystal structure analysis were introduced by B. Zvyagin, for polycrystalline, mostly inorganic crystals and by J. Fryer and B. Jap for organic and biological materials. The use of direct methods for structure analysis with electrons was described by D. Dorset, and J. Fryer gave an introduction to the use of maximum entropy techniques. The considerable progress made in combining the information from high resolution electron microscopy (phase information but limited resolution) and electron diffraction (amplitude information only but much higher resolution) was amply illustrated by F. H. Li for inorganic materials and by D. Typke for biological samples.

An excellent account of dynamical diffraction theory by J. Spence, preceded a discussion of recent real-space and mixed-space approaches and channeling theory by D. Van Dyck. Bethe-type dynamical diffraction was introduced by J. Spence before J. Zuo described its application in convergent-beam electron diffraction (CBED). The use of specifically dynamical diffraction effects for the highly accurate determination of structure factors was described by J. Zuo and R. Hoier and by S. Matsumura who concentrated on the critical voltage technique. J. Spence described the role of inelastic scattering processes, in particular in relation to the determination of crystal polarities.

The general introduction to the power of CBED methods by J. Steeds was followed by J. Tafto's account of the development of large-angle convergent-beam electron diffraction (LACBED) which is proving highly productive in the analysis of complicated crystal structures. S. Hovmoeller provided demonstrations and instruction on the use of a small computer in the analysis of electron-diffraction patterns and the interpretation of high-resolution electron microscope images of crystals.

A lively panel-discussion session led by J. Spence focused largely on the requirements for quantitative data collection. An unresolved question was why kinematical-theory-based analysis of crystal structures should be so successful when the dynamical diffraction effects are so pervasive in electron diffraction. The answer may come at the Summer School in Bristol next year.

The efforts of Z. Zhang, B-S. Cao, L-M. Peng of the Beijing Laboratory of Electron Microscopy, Academy of Sciences, and K-H. Kuo, the President of Tsinghua U. to make this a memorable meeting were extremely successful and greatly appreciated.

J. M. Cowley